Why some objects are not mirrored?
2 ways you can mirror

1st method : If yes then delete the half. And the part which you want to mirror is loose or say separate parts so join Ctrl+L and make it one mesh and then mirror it.

or if you do not want to join and keep separate 

2nd method : Just group the half you want to mirror. with selected half group press shift+D and without moving just rotate 180 on x axis. (As the 3d cursor is set)